Single-Cell and Single-Molecule Studies of Bacteriophage Lambda Post-Infection Decision-Making
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Cellular decision making is a ubiquitous process among all life forms, and a key step that organisms take to integrate the environmental signals to choose an optimal response to improve their overall fitness. The genetic circuits selected to carry out this task determine the cell fate in a seemingly probabilistic way, either due to the inherent stochasticity of the system, or our inability to characterize the factors with deterministic impacts. To gain a better understanding of the mechanisms underlying cell-fate selection, we utilize a well-established system for cellular decision-making, the paradigm of bacteriophage lambda infection, which leads to two distinct outcomes – lysis and lysogeny. Recent studies of this system using higher resolution techniques suggested that different phage decisions are partially determined by pre-existing difference and the complex in vivo phage-phage interactions. Therefore, characterizing more ‘hidden’ deterministic factors and dissecting the intracellular behaviors of phage components, such as DNA, RNA and proteins are central to a more complete understanding of the phage decision-making strategies. One commonly overlooked but potentially important factor is phage DNA replication, which could result in not only more templates for gene expression but also introduce gene copy number variations. Meanwhile, although theoretical work has long predicted that noise arising from stochastic gene expression can be propagated through the gene networks to result in phenotypic variance, experimental characterization is still lacking, impeding the assessment of its contributions to phage decision-making. In this work, we provided direct experimental evidence that different phage DNAs are capable of making decisions independently. DNA integration, a characteristic event for phage lysogenization, can also be detected in lytic cells. Moreover, through single phage DNA labeling technique, we revealed great heterogeneity in intracellular DNA motions, which could partially explain the complex phage-phage interactions. Furthermore, we found that DNA replication is important for the enforcement of decisions. Instead of affecting the transcription of early lysis-lysogeny decision-making genes, DNA replication exerts its effect on the expression of the decision effectors, CI. Lastly, a mathematical model is built to provide comprehensive understanding of the decision making network.
Shao, Qiuyan (2017). Single-Cell and Single-Molecule Studies of Bacteriophage Lambda Post-Infection Decision-Making. Doctoral dissertation, Texas A & M University. Available electronically from